This application claims the priority of German patent document 199 09 933.2, filed Mar. 6, 1999, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method and apparatus for exhaust gas purification, which provides an ammonia generating catalyst for generating ammonia from nitrogen oxides contained in the exhaust gas to be purified, and which includes a nitrogen oxide reducing catalyst arranged downstream of the ammonia generating catalyst for reducing the nitrogen oxides contained in the exhaust gas to be purified, using the generated ammonia as the reducing agent.
Exhaust gas purification systems and operating methods of this type are described in International Published Patent Application WO 97/17532 A1, and are used particularly for purifying the exhaust gas of a preferably multi-cylinder motor vehicle internal-combustion engine. The exhaust gas of each cylinder is fed via a short exhaust gas connection to a common exhaust pipe in which all exhaust-gas-purifying components are situated.
In one of the embodiments disclosed there, a three-way ammonia generating catalyst, a nitrogen oxide adsorption catalyst which simultaneously operates as a nitrogen oxide reducing catalyst, and an ammonia conversion catalyst are connected sequentially in the exhaust gas flow direction. The engine is operated in a time-alternating manner, with a rich (high-fuel) and a lean (low-fuel) mixture of fuel and air. In the lean operating phases, the nitrogen oxides are intermediately stored in the nitrogen oxide adsorption catalyst. In the rich operating phases, ammonia is generated by the ammonia generating catalyst from the nitrogen oxides contained in the exhaust gas; the generated ammonia then causes a nitrogen oxide reduction in the nitrogen oxide adsorption catalyst, which follows, with the desorption of the nitrogen oxides previously adsorbed there. Any excess ammonia is reduced in the ammonia conversion catalyst which follows.
In a further embodiment, instead of the nitrogen oxide adsorption catalyst, an ammonia adsorption catalyst is provided which simultaneously operates as a nitrogen oxide reduction catalyst. In the rich operating phases, the ammonia generated by the ammonia generating catalyst is intermediately stored in the ammonia adsorption catalyst. In the lean operating phases, the nitrogen oxides contained in the exhaust gas are reduced while the ammonia is desorbed in the ammonia adsorption catalyst.
It still another embodiment, the nitrogen oxide adsorption catalyst and the ammonia adsorption catalyst are serially situated between the ammonia generating catalyst and the ammonia conversion catalyst. In the lean operating phases, the nitrogen oxides contained in the exhaust gas are intermediately stored in the nitrogen oxide adsorption catalyst. Nitrogen oxides which may not have been adsorbed there are reduced in the subsequent ammonia adsorption catalyst, with the desorption of ammonia. In the rich operating phases, the ammonia generating catalyst generates ammonia from the nitrogen oxides contained in the exhaust gas; this ammonia causes a desorption and reduction of intermediately stored nitrogen oxides in the subsequent nitrogen oxide adsorption catalyst. In all implementations, it is endeavored for reasons of fuel consumption, to enable the engine to drive as long as possible in the lean operation.
One object of the present invention is to provide a method and apparatus for exhaust gas purification of the type described above, by means of which nitrogen oxides contained in the exhaust gas can be converted as effectively as possible by internally generated ammonia.
Another object of the invention is to provide such a purification method and apparatus in which the one or several combustion sources generating the exhaust gas to be cleaned can be operated with a fuel consumption which is as low as possible.
These and other objects and advantages are achieved by the exhaust gas purification system according to the invention, which is suitable specifically for purifying the exhaust gas of at least two separately controllable groups of one or more internal-combustion sources (e.g, engine cylinders). A separate exhaust pipe branch arranged parallel to one (or the other) exhaust pipe branch(es) of the other groups is assigned to at least a first group; and all exhaust pipe branches jointly lead to the nitrogen oxide reducing catalyst. An ammonia generating catalyst is characteristically arranged in the exhaust pipe branch which is part of the first group of combustion sources, and is therefore acted upon only by the partial exhaust gas flow of this group of combustion sources. The other internal-combustion sources can therefore be operated independently of the requirements of the ammonia generating catalyst. On the other hand, the operation of the group of combustion sources pertaining to the ammonia generating catalyst can be adapted specifically to meet its requirements.
In one embodiment of the invention, in addition to the ammonia generating and nitrogen oxide reducing catalysts, the exhaust gas purification system has a nitrogen oxide adsorption catalyst which is characteristically arranged upstream of the ammonia generating catalyst. As a result, nitrogen oxides which are increased in the exhaust gas in lean operating phases of the combustion sources can be intermediately stored in the nitrogen oxide adsorption catalyst, and can be desorbed in a respective subsequent rich operating phase and, in the ammonia generating catalyst, which follows, can be utilized at least partially for generating ammonia.
Another embodiment of the invention combines the above measures in that a nitrogen oxide adsorption catalyst and an ammonia generating catalyst are connected in sequence in an exhaust pipe branch, which is part of one of several groups of combustion sources. The latter exhaust pipe branch, together with one or more exhaust pipe branches pertaining to the other combustion sources, leads to nitrogen oxide reducing catalyst.
In another embodiment, the group of combustion sources, whose partial exhaust gas flow acts upon the ammonia generating catalyst (and optionally the nitrogen oxide adsorber catalyst connected on the input side) is operated continuously or at least at times in the rich operation. This ensures the formation of a sufficient amount of ammonia in order to be able to reduce also the nitrogen oxides contained in the exhaust gas of the other internal-combustion sources in the nitrogen oxide reducing catalyst which follows, while utilizing the ammonia as a reducing agent.
In still another embodiment of the invention, the one or more other combustion sources, whose exhaust gas bypasses the ammonia generating catalyst, are continuously operated in the lean operation mode in order to keep the fuel consumption as a whole as low as possible.
According to yet another embodiment of the invention, the one or more combustion sources, whose exhaust gas acts upon the nitrogen oxide adsorption catalysts, are periodically operated alternately in the lean and rich regions. In this case, the individual operating phases can be controlled such that, on the one hand, the fuel consumption remains as low as possible and, on the other hand, sufficient ammonia is always provided as a reducing agent for reduction of nitrogen oxide.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.